This invention relates to a protection apparatus and a protection method of a PWM cycloconverter for converting an AC voltage into an arbitrary AC voltage or DC voltage.
FIG. 8 shows a protection circuit for suppressing a surge voltage of the output side of a PWM cycloconverter in a related art. In FIG. 8, numeral 1 denotes a three-phase AC power supply, numeral 100 denotes a normal PWM cycloconverter, numeral 21 denotes a three-phase AC motor, numeral 110 denotes a rectification snubber circuit, numeral 120 denotes an output short circuit, numerals 111, 112, and 121 denote three-phase rectification bridges, numerals 113 and 122 denote uni-directional semiconductor switches, 114 and 115 denote resistors, numeral 116 denotes a capacitor, numeral 117 denotes a voltage detection circuit, and numeral 118 denotes a gate driver. In the PWM cycloconverter, the output and input sides are connected directly by bi-directional semiconductor switches for allowing a current to flow in bi-directionals and thus if two or more of three bi-directional switches connected in the same output phase are turned on at the same time at the commutating time, the input side becomes short-circuited and if the three bi-directional switches are turned off at the same time, the output side becomes open. As means for preventing the phenomenon, the load current direction is detected and the information can be used to commutate without opening the output side and without short-circuiting the input side. However, if the load current direction is erroneously detected during commutating (easy to be erroneously detected particularly in the vicinity of zero of load current), there is a possibility that the output side may be opened. When an overcurrent or an overvoltage on the input side or the load side, overheating of a motor winding or a semiconductor device of the main circuit, or the like (simply, operation anomaly) occurs, it is necessary to shut off the current of the PWM cycloconverter, namely, stop the operation abruptly. If all three bi-directional switches are turned off at the same time, the output side becomes open. To drive an induction motor, if the output side of the PWM cycloconverter becomes open during the operation, a large surge voltage occurs on the output side of the PWM cycloconverter.
When the output side of the PWM cycloconverter becomes open because of a commutation mistake during the normal operation, the load current continues to flow into the capacitor 116 via the three-phase rectification bridge 112, and a large surge voltage does not occur. The voltage detection circuit 117 detects the voltage level of the capacitor 116 and when the voltage of the capacitor 116 becomes a certain degree or more, the voltage detection circuit 117 outputs an overvoltage signal OV. The uni-directional switch 113 is turned on by the overvoltage signal OV and the capacitor 116 is discharged. When an operation anomaly occurs, first all uni-directional switches of the PWM cycloconverter are turned off. After a given time interval (xcex94t) from the time, the uni-directional switch 122 is turned on. xcex94t should be longer than the on or off time of the semiconductor switch. The load current continues to flow into the capacitor 116 via the three-phase rectification bridge 112 during xcex94t and the uni-directional switch 122 is turned on and thus the load current continues to flow via the three-phase rectification bridge 121 and the uni-directional switch 122 and reduces to zero.
However, in the protection method of the PWM cycloconverter in the related art, if the uni-directional switch 122 is turned on during the operation and three phases of output are short-circuited for a long period of time (for example, until the end of shut off), an excessive motor stator current flows and there is a possibility that the facilities may be destroyed.
Next, the function of rectification snubber is to absorb a surge voltage occurring in the period of time in which the output side becomes open. The output side becomes open in either of the following two cases: One case where the load current direction is erroneously detected at the commutating time and the other case being the time interval xcex94t until all uni-directional switches are turned off and the uni-directional switch 122 is turned on at the shut off time. In both cases, the period of time in which the output side becomes open is almost the same; however, as compared with the fact that a detection mistake of the load current direction occurs only in the vicinity of zero of the load current, shut off caused by a fault, etc., often occurs when the current is rated current or overcurrent. Therefore, the surge voltage in the latter case is larger by far than that in the former case and as its countermeasure, the capacitor capacitance of the rectification snubber is large; this is also a problem. Further, normally the surge voltage produced by a commutation mistake is absorbed by the capacitor 116 of the rectification snubber and electric power accumulated in the capacitor 116 of the rectification snubber cannot be regenerated; this is also a problem.
It is therefore an object of the invention to provide a protection apparatus and a protection method of a PWM cycloconverter wherein when the PWM cycloconverter is shut off, a surge voltage is not produced on the output side, an overcurrent of a motor stator is not produced, an output short circuit is not required, and moreover a rectification snubber circuit does not exist or the capacitance of a capacitor of rectification snubber can be lessened exceptionally.
To solve the above-mentioned problems, according to the invention, there is provided a protection apparatus of a PWM cycloconverter of a power converter wherein phases of a three-phase AC power supply and phases of the power converter for outputting M phases (where M is an integer of two or more) are directly connected by bi-directional semiconductor switches each consisting of a pair of uni-directional semiconductor switches for allowing an electric current to flow only in uni-directional and being able to be turned on and off separately, the protection apparatus comprising:
fault detection means for detecting a fault of the above-mentioned PWM cycloconverter;
operation stop means for turning off at least some of the above-mentioned uni-directional semiconductor switches of the above-mentioned PWM cycloconverter if the above-mentioned fault signal occurs;
continuous flow means for allowing a load current of the above-mentioned PWM cycloconverter to continuously flow if at least some output phases of the above-mentioned PWM cycloconverter become open; and
protection switching means for turning on and off the above-mentioned uni-directional semiconductor switches of the above-mentioned PWM cycloconverter based on the fault signal output from the above-mentioned fault detection means, thereby allowing the above-mentioned load current to continuously flow into the above-mentioned three-phase AC power supply via the above-mentioned PWM cycloconverter.
In the invention, the above-mentioned protection switching means comprises:
input voltage information detection means for recognizing voltages of three terminals of the input side of the above-mentioned bi-directional semiconductor switches as three-phase input voltages and detecting information based on which greater-than, equal-to, less than relation of the above-mentioned three-phase input voltages can be determined as input voltage information and regenerative switching means for turning on only the above-mentioned uni-directional semiconductor switch from each output phase to an input phase which becomes the maximum voltage of the above-mentioned three-phase input voltages or only the above-mentioned uni-directional semiconductor switch from an input phase which becomes the minimum voltage of the above-mentioned three-phase input voltages to each output phase based on the input voltage information.
In the invention, the above-mentioned protection switching means comprises the above-mentioned input voltage information detection means and the above-mentioned regenerative switching means;
short-circuit switching means for turning on at least some of the above-mentioned unidirectional semiconductor switches between one arbitrary phase of the above-mentioned three-phase AC power supply and each output phase; and
switch means for switching the above-mentioned regenerative switching means and the above-mentioned short-circuit switching means for use.
In the invention, the protection apparatus of the PWM cycloconverter comprises:
input voltage information detection means for recognizing voltages of three terminals of the input side of the above-mentioned bi-directional semiconductor switches as three-phase input voltages and detecting information based on which greater-than, equal-to, less than relation of the above-mentioned three-phase input voltages can be determined as input voltage information and gate drive means for always turning on the above-mentioned uni-directional semiconductor switch from each output phase to an input phase which becomes the maximum voltage of the above-mentioned three-phase input voltages and the above-mentioned uni-directional semiconductor switch from an input phase which becomes the minimum voltage of the above-mentioned three-phase input voltages to each output phase based on the input voltage information while the above-mentioned PWM cycloconverter is operating or is shut off.
According to the invention, there is provided a protection method of a PWM cycloconverter of a power converter wherein phases of a three-phase AC power supply and phases of the power converter for outputting M phases (where M is an integer of two or more) are directly connected by bi-directional semiconductor switches each consisting of a pair of unidirectional semiconductor switches for allowing an electric current to flow only in uni-directional and being able to be turned on and off separately, the protection method comprising the steps of
detecting a fault of the above-mentioned PWM cycloconverter;
turning off at least some of the above-mentioned uni-directional semiconductor switches of the above-mentioned PWM cycloconverter if the above-mentioned fault signal occurs;
allowing a load current of the above-mentioned PWM cycloconverter to continuously flow if at least some output phases of the above-mentioned PWM cycloconverter become open; and
turning on and off the above-mentioned uni-directional semiconductor switches of the above-mentioned PWM cycloconverter based on the fault signal output from the above-mentioned fault detection means, thereby allowing the above-mentioned load current to continuously flow into the above-mentioned three-phase AC power supply via the above-mentioned PWM cycloconverter.
In the invention, voltages of three terminals of the input side of the above-mentioned bi-directional semiconductor switches are recognized as three-phase input voltages and information based on which greater-than, equal-to, less than relation of the above-mentioned three-phase input voltages can be determined is detected as input voltage information and only the above-mentioned uni-directional semiconductor switch from each output phase to an input phase which becomes the maximum voltage of the above-mentioned three-phase input voltages or only the above-mentioned uni-directional semiconductor switch from an input phase which becomes the minimum voltage of the above-mentioned three-phase input voltages to each output phase is turned on based on the input voltage information.
In the invention, regenerative switching means for turning on only the above-mentioned uni-directional semiconductor switch from each output phase to an input phase which becomes the maximum voltage of the above-mentioned three-phase input voltages or only the above-mentioned unidirectional semiconductor switch from an input phase which becomes the minimum voltage of the above-mentioned three-phase input voltages to each output phase based on the input voltage information and short-circuit switching means for turning on at least some of the above-mentioned uni-directional semiconductor switches between one arbitrary phase of the above-mentioned three-phase AC power supply and each output phase are switched.
In the invention, in the protection method of the PWM cycloconverter,
voltages of three terminals of the input side of the above-mentioned bi-directional semiconductor switches are recognized as three-phase input voltages and information based on which greater-than, equal-to, less than relation of the above-mentioned three-phase input voltages can be determined is detected as input voltage information and
the above-mentioned uni-directional semiconductor switch from each output phase to an input phase which becomes the maximum voltage of the above-mentioned three-phase input voltages and the above-mentioned uni-directional semiconductor switch from an input phase which becomes the minimum voltage of the above-mentioned three-phase input voltages to each output phase are always turned on based on the input voltage information while the above-mentioned PWM cycloconverter is operating or is shut off.
As described above, to protect the PWM cycloconverter, according to the invention, the following advantages are provided: The output short circuit 120 is not required, the capacitor of the rectification snubber can be lessened, and when the PWM cycloconverter is shut off, a surge voltage does not appear on the output side and the motor current is not increased. The advantages that the rectification snubber circuit is not required and that the power loss caused by a commutation mistake is small are also provided.